Shock absorbing devices are used in a wide variety of vehicle suspension systems for controlling motion of the vehicle and its tires with respect to the ground and for reducing transmission of transient forces from the ground to the vehicle. Shock absorbing struts are a common and necessary component in most aircraft landing gear assemblies. The shock struts used in the landing gear of aircraft generally are subject to more demanding performance requirements than most if not all ground vehicle shock absorbers. In particular, shock struts must control motion of the landing gear, and absorb and damp loads imposed on the gear during landing, taxiing and takeoff.
A shock strut generally accomplishes these functions by compressing a fluid within a sealed chamber formed by hollow telescoping cylinders, where at least two bearings provide for sliding engagement of the telescoping cylinders. Typically, the lower bearing is fixed to the outer cylinder and provides for sliding engagement with the inner cylinder or piston, while the upper bearing is fixed to and moves with the piston, providing for sliding engagement with the outer cylinder.
The fluid within the sealed chambers generally includes both a gas and a liquid, such as hydraulic fluid or oil. One type of shock strut generally utilizes an “air-over-oil” arrangement wherein a trapped volume of gas is compressed as the shock strut is axially compressed, and a volume of oil is metered through an orifice. The gas acts as an energy storage device, such as a spring, so that upon termination of a compressing force the shock strut returns to its original fully-extended length. Shock struts also dissipate energy by passing the oil through the orifice so that as the shock absorber is compressed or extended, its rate of motion is limited by the damping action from the interaction of the orifice and the oil.
Designers of aircraft landing gear are always looking for ways to improve design, performance, cost, etc, while dealing with issues related to wear and corrosion. One desirable improvement is reducing the overall weight of the shock strut within the landing gear. While some efforts have been made to incorporate lighter weight materials into shock struts, it has been the case that lighter weight materials are more susceptible to premature wear, thereby requiring frequent replacement and resulting in increased maintenance costs.